Why is my render turning out grey?
I think I have found your problem. I have a very fast computer as well, and the same grey thing happens too, but not very often. To trigger the grey, I had to move around very fast and "use up a lot of power". As fast as your computer may seem, it still has a max.

But here are some things you could try:


File > User Preferences > System > and check "VBOs".
Try to moving some objects to different layers, oddly enough Blender calculates each layer separately. (There is an article about that: http://www.blenderguru.com/articles/how-to-render-a-complex-scene-without-crashing/)
If all else fails, you could try reducing the number of complicated materials and vertices.


Hope that helps!